Wireless communications networks allow users to communicate over an air interface through the use of client devices. In a typical wireless network, a radio access network (“RAN”) facilitates client devices communicating over the air interface. A RAN may be communicatively coupled to other types of networks, such as the public switched telephone network (“PSTN”) or a packet switched network like the Internet. The connectivity between these networks allows users of client devices to engage in many different types of communications, including telephone calls with cellular telephones, telephone calls with landline telephones, and the transmission or receipt of electronic mail messages.
A RAN may include, among other components, base transceiver stations (“BTSs”), radio network controllers (“RNCs”), and switches or gateways, such as mobile switching centers (“MSCs”) and packet data serving nodes (“PDSNs”). A BTS, also known as a base station, comprises a cell tower with one or more antennas that radiate to define a cell and cell sectors. A BTS may serve client devices within the geographic coverage area corresponding to its cell, such that client devices within that area receive signals from and transmit signals to the BTS. An RNC may control one or more BTSs and thus control various aspects of air interface communication. An RNC may also route communication signals from BTSs to other networks. For instance, the RNC may route communications to an MSC, which may provide connectivity with the PSTN. Similarly, the RNC may route communications to a PDSN, which may provide connectivity with the Internet or other packet switched network.
A vocoder is a network component that may encode signals to be transmitted by an entity and decode signals that have been received by the entity. In a typical wireless communication network, an RNC and a client device may each have its own vocoder. The vocoder of a client device may digitize, compress, or otherwise encode a voice signal received from a user for transmission over an air interface to a BTS. The BTS may relay the signal from the client device to the RNC, and the vocoder of the RNC may decode the encoded voice signal. The RNC may then send the decoded representation of the voice signal to a network switch or gateway for transmission to another endpoint. If the RNC receives a signal from a remote device to be transmitted to the client device, the vocoder of the RNC may encode that signal. The RNC may then transmit the encoded signal through the BTS to the client device, and the vocoder of the client device may then decode the signal so that the signal may be communicated to the user of the client device.
Vocoders in wireless wide area networks, such as the RANs described above, commonly use a number of different compressed digital formats, which are well known to those of ordinary skill in the art, to encode and decode signals transmitted over an air interface. One exemplary format is the Enhanced Variable Rate Code (“EVRC”), which supports voice transmission at a data rate of about 8 kbps. EVRC is described in the TIA/EIA/IS-127-3 standard. “Enhanced Variable Rated Codec, Speech Service Option 3 for Wideband Spread Spectrum Digital Systems.” Another example is the Selectable Mode Vocoder (“SMV”), which supports voice transmission at a data rate of about 4 kbps. SMV is described in the TIA/EIA/IS-893 standard, “Selectable Mode Vocoder Service Option for Wideband Spread Spectrum Communication Systems.” Only requiring about half the bandwidth of EVRC, SMV is a higher compression format than EVRC. Wireless devices that support the SMV format are typically backward compatible; thus, these devices likely support the less efficient EVRC format as well as the SMV format.